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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

GLACIATION OF THE YAKUTAT BAY REGION                 213
many hundreds of large icebergs floating in the fiord at all times, most of "which must have come from the lower part of the glacier. When such great masses rise they send out huge waves which cause violent surf on the neighboring coast.
The formation and dispersion of icebergs produces important results in addition to checking the advance of glaciers. One of the most important of these is that of marine erosion resulting from the iceberg waves. Within three or four miles of Hubbard Glacier front iceberg waves are almost constantly breaking, and frequently with such violence that it is sometimes difficult to land, even on a beach, and dangerous to attempt a landing on a rocky coast. These waves are accomplishing far more work of erosion than the wind waves, and they are both building beaches and cutting sea cliffs. Gilbert found, what our observations abundantly confirm, that some of the sea cliffs near the tidal glacier fronts are at a higher level than these waves now reach, evidently having been formed when the glacier fronts were nearer and the waves, therefore, both higher and more vigorous.
Since practically all of the icebergs melt within Yakutat Bay they are important agents in transportation of the sediment which is accumulating there. They contribute both to the beaches and to the sediments away from the shore, which are mainly clay brought by the glacial streams. One may be confident that scattered through these fine-grained sediments is a notable admixture of coarser ice-borne fragments, even to the size of large bowlders. By this iceberg dispersion of glacier-borne material, the rate of upbuilding of moraine deposits at the front of tidal glaciers is greatly diminished; and when one finds such an extensive submarine moraine as that which sweeps in a broad crescent at the mouth of Yakutat Bay he may feel certain that the ice stood there for a long time. Probably, too, it is composed in large part of materials poured into the sea by the glacial streams.
Many of the icebergs are stranded on the beaches. At times the west shore of Disenchantment Bay and Yakutat Bay as far as the Kwik River is so covered by stranded bergs that it is difficult to land a boat, especially when the surf is breaking. These stranded icebergs melt rapidly and contribute much material to the beaches; the surf, swirling among them, moves and grinds up the beach material; the icebergs are rocked and pushed back and forth, aiding directly in erosion; and only rarely do they form such a rampart as to check erosion by completely breaking the force of the waves. Off shore, large bergs are often stranded, and when they run aground, they must plow up the bottom material, while when they break and the fragments rock back and forth, they must cause still further erosion, and the waves to which they give rise add materally to the surf work on the neighboring beaches. Many of the stranded bergs remain for several days in one place and must give rise to local deposits of coarse material, forming pockets in the midst of plowed up and disturbed clay and sand sediments. The same phenomenon is often observed on the beaches where icebergs have stranded at high tide, and on melting, or floating off, have left pockets of angular rock fragments in a pit in the sand, where the iceberg stood. So much coarse material is brought by the icebergs that all the beaches in the zone of abundant bergs contain a notable admixture of coarse fragments, including bowlders of good size, especially at and near the low tide mark, where large icebergs often strand.
Dispersion of the Icebergs. The dispersion of the icebergs of Yakutat Bay is very peculiar, for, notwithstanding their abundance and the large size of many of them, it